Process for manufacturing hot-dip aluminized pearlitic malleable cast iron and black heart malleable cast iron products

ABSTRACT

A process for manufacturing hot-dip aluminized pearlitic malleable iron castings with lamellar or coagulated cementite and also black heart malleable iron castings by carrying out the graphitizing annealing simultaneously with the annealing giving a homogeneous coating on aluminized castings. This process of the invention consisting in annealing the castings with an aluminium protective coating enables eliminating oxidation of their surfaces as well as decarbonization of the surface layer, providing thereby the homogeneity of the structure of the castings and also the homogeneous structure of the aluminized coating, avoiding moreover the surface scorches of quartz sand, which is used as a filler in the hitherto known processes.

United States Patent 1191 Gierek et a1. 9

[ Nov. 26, 1974 1 1 PROCESS FOR MANUFACTURING HOT-DIP ALUMINIZED PEARLITIC MALLEABLE CAST IRON AND BLACK HEART MALLEABLE CAST IRON PRODUCTS [75] Inventors: Adam Gierek; Lech Bajka, both of Katowice, Poland 73 Assignee: Politechnika Slaska 1M. W.

Pstrowskiego, Gliwice, Poland [22] Filed: Sept. 11, 1972 [21] Appl. No.1 288,161

[30] Foreign Application Priority Data Sept. 15. 1971 Poland 1. 150514 [52] U.S. C1 l48/6.11, 143/139, 29/1962, 148/138, 148/141, 148/627 [51] int. Cl C23C 1/00, C23f 5/02, CZ3C 1/08 [58] Field of Search 148/6, 6.11, 6.27, 138-141, 148/315, 35; 29/1962 1 [56] References Cited UNITED STATES PATENTS 2,887,421 5/1959 PCI'ZIS 148/139 3,268,369 8/1966 Hangen 29/1962 Primary Examiner-John D. Welsh Attorney, Agent, or Firm-Waters, Roditi, Schwartz & Nissen [57] ABSTRACT A process for manufacturing hot-dip aluminized pearlitic malleable iron castings with lamellar or coagulated cementite and also black heart malleable iron castings by carrying out the graphitizing annealing simulta' neously with the annealing giving a homogeneous coating on aluminized castings.

hitherto known processes.

7 Claims, 5 Drawing Figures PATENTELIIBVZBIW 3,850,699

SHEET 2 BF 3 PROCESS FOR MANUFACTURING HOT-DIP ALUMINIZED PEARLITIC MALLEABLE CAST IRON AND BLACK HEART MALLEABLE CAST IRON PRODUCTS The invention relates to a process for manufacturing hot-dip aluminized pearlitic malleable iron castings with lamellar or coagulated cementite and also black heart malleable iron castings. These products are provided for working at elevated temperatures and in highly corrosive conditions.

The production of hot-dip aluminized iron castings comprises two separate technical processes, namely a process for producing iron castings and a process for hot-dip aluminizing these iron castings. One of the known processes for producing pearlitic malleable iron castings with lamellar or spheroidal cementite is based on the successive operations of heat treatment of the previously produced white cast iron castings being as a rule hypo-eutectic.

The white cast iron castings are annealed at temperatures in the range of 900 1000C for 8 16 hours in an inert atmosphere in order to graphitize them, effecting thereby decomposition of free cemetite and isolation of the temper carbon. After complementing the annealing, the castings together with the furnace are cooled to the range of temperatures of the eutectoid transformation i.e. from 850 730C and subsequently fast cooled to the ambient temperature.

The graphitizing process at temperatures from 900 lOOC may be preceded by a preliminary quenching of the white cast iron castings or by preliminary quenching with tempering. Applying the operation of the preliminary quenching or quenching combined with tempering greatly increases the quantity of graphitization nuclei causing thereby the refinement of temper carbon in the structure of the matrix of the pearlitic malleable cast iron or malleable black heart cast iron which greatly increases the mechanical properties of these cast irons.

If it is required to produce the pearlitic structure with spheroidal cementite, the obtained castings having a pearlitic structure with lamellar cementite are submitted to subsequent annealing at temperatures from 6507 80C for the period of 0.25 hours. The said operation causes the coagulation of cementite which appears in pearlite in the form 'of lamellas. After complementing the process of annealing, the castings are slow cooled and taken out of the furnace.

There is also the possibility of producing materials from a perlitic malleable cast iron with coagulated cementite by annealing the previously quenched black heart malleable ironcastings in the range of temperatures of 650 800C for the period from 0.25 10 hours and subsequently cooling them to the ambient temperature. In order to prevent corrosion of the surpoint when the dipped castings reach the temperature of the bath- As a result of this process, a protective coating is produced on surfaces ofaluminized castings, consisting of several layers which comprise from outside: a layer of aluminum oxides, aluminium or aluminium alloys having a chemical composition the same as the composition of the bath, the mixture of aluminium and iron compounds with aluminium, inter-metallic phases of aluminium and cast iron and the solid solution of aluminium inthe alpha iron. The multilayer coating as well as high brittleness of the layer of intermetallic phases have a bad effect on its mechanical properties. Therefore after hot-dip aluminizing, the castings should be annealed at high temperatures in the range of 700 1,200C for a period of 4-20 hours. As a result the multilayer coating is transformed partially or entirely into a solid solution of aluminium in the alpha iron, which is characterized by an outstanding higher plasticity and corrosion resistance increased and additionally by the formation of a thin layer of aluminium oxides on the surface of the annealed and previously aluminized castings.

However the hitherto known processes for manufacturing hot-dip aluminized pearlitic malleable castingsand black heart malleable castings show several serious disadvantages.

The process for producing black heart and pearlitic malleable castings and the subsequent process of diphot aluminizing them are carried out separately i.e. the aluminizing operation is applied after complementing the graphitizing annealing, which according to the known method has to be carried out in an inert medium. The inert medium can be obtained by using protective atmospheres or by placing the annealed castings in annealing pots and ridging them with an inert material, most frequently with quartz sand. The annealing in protective atmospheres i.e. in a chemically neutral or reducing medium involves disadvantageous surface changes such as adsorption of particles of protective gases on the surface of the products and resorption of some components of the cast iron causing thereby oxidation of the surface. The adsorption causes also a partial decarbonization of the surface layer and when a malleable black heart iron is used, the formation of a thin layer of pearlite or ferrite near the surface /a light lining/effecting a considerable decrease of plasticity of the worked castings. On the other hand, the castings annealedin annealed pots filled with quartz sand undergo surface decarbonization or oxidation as a result of adsorption of particles of the gaseous phase. In the course of annealing at temperatures of 950 1,000C on the surface of castings, a layer of oxides is formed, which in the presence of grains of quartz sand, being subjected to allotropic changes causes the formation of new minerals on their surfaces in the form of visible, hardly removable scorches, which considerably complicates dressing the surfaces of annealed castings before the hot-dip aluminization.

The present invention aims at reducing and overcoming the disadvantages of the hitherto known processes for producing hot-dip aluminized pearlitic malleable and black heart malleable castings and also aims at obtaining products having a homogeneous structure, free from surface decarbonization and oxides and scorches, covered with a structurally homogeneous coating formed by hot-dip aluminizing thereby providing high mechanical properties of the produced cast irons as well as high heatand corrosion resistances, simultaneously with considerable shortening of annealing rates, which can proceed in a neutral, oxidizing or reducing medium.

This problem has been solved by carrying out the graphitizing annealing step simultaneously with the annealing step for providing a homogeneous coating on aluminized white cast iron castings, black heart malleable castings or pearlitic malleable castings in an optional atmosphere. The said process consisting in annealing the castings with an aluminium protective coating, enables the elimination of oxidation of their sur faces as well as decarbonization of the surface layer, thereby providing homogeneity of the structure of the castings and also a homogeneous structure of the aluminized coating, avoiding moreover surface scorches of quartz sand, which is used as a filler in the hitherto known processes.

The additional advantages attained in this process of the invention for producing hot-dip aluminized black heart and pearlitic malleable castings are: shortening of the annealing rate by simultaneous annealing of the core and the coating, elimination of expensive and complicated annealing furnaces with protective atmospheres and also fast-wearing annealing pots, These improvements enable elimination of laborious operations such as dressing of the surfaces of annealed castings before aluminizing them, loading the castings into the annealing pots and ridging them with sand which as well as the inert gases is not used in the process of the invention.

The said process is therefore fast, simple, economical and results in higher operational efficiency than has been heretofore realizable.

Example I.

A process of the invention for producing hot-dip aluminized castings from a malleable pearlitic cast iron with a lamellar cementite consists in heat treatment of castings from white cast iron/in order to obtain the pearlitic structure with a lamellar cementite and isolate the temper carbon or to obtain the pearlite-ferritic structure with a temper carbon/simultaneously with dip aluminizing these castings and subsequently annealing them in order to obtain the homogeneity of the multilayer aluminium coating formed directly after drawing them out from the bath. The castings from white cast iron with accurately dressed surfaces are immersed in the bath containing aluminium or aluminium alloys at temperatures in the range of 680 950C at a rate from 0.5 5 m/min. The castings are kept in the bath for the time period which is necessary for them to obtain the bath temperature. Then the castings are taken out from the bath at a rate from 0,5 m/min and cooled on air or in water This operation enables the castings to obtain a protective aluminium coating on the their surfaces a simultaneous hardening by formation of graphitization seeds in great numbers thereby increasing the final mechanical properties of the aluminized products from a malleable pearlitic cast iron with a lamellar cementite. Subsequently, graphitizing annealing is effected in a range of temperatures of 900 1,000C for the period of 8 l6 hours. This operation is performed in order to additionally increase the quantity of graphitization seeds and it may be preceded by tempering of previously hardened castings. Simultaneously with the graphitizing annealing of the aluminized castings the formed aluminium coating is also annealed in order to obtain improved structure. After the annealing stage, the aluminized castings together with the furnace are cooled to the critical range of temperatures from 880 730C and then fast cooled in air to the ambient temperature. This course of the process ensures a high efficiency in the production of aluminized castings from a pearlitic malleable cast iron with a lamellar cementite. This process is illustrated in the diagram presenting the process rate in dependence on the range of temperatures of successive heat treatments as is shown in FIG. 1.

Example 2. In order to produce hot-dip aluminized castings from a pearlitic malleable cast iron with a spheroidal cementite, the heat treatment of previously hot-dip aluminized castings from a white cast iron is carried out with the same parameters as in the process for producing hot-dip aluminized castings from a pearlitic malleable cast iron with a lamellar cementite, and after the stage of graphitizing annealing an additional annealing is effected in the temperature range from 650780C for a period of 0.25 10 hours. This operation enables obtaining a better pearlite structure with coagulated cementite and further annealing of formed coating. The process is shown in FIG. 2 which illustrates the rate of the process in dependence on the range of temperatures of the heat treatment.

Example 3. The hot-dip aluminized castings from a pearlitic malleable cast iron with a spheroidal cementite are also obtained from a pearlitic malleable cast iron with a lamellar cementite, having no protective coating but only a dressed surface. They are hot-dip aluminized with the same peremeters as in the process of hot-dip aluminizing described in Example I. The castings with a formed aluminium coating are annealed for a period of 0.25 10 hours in a temperature range of 650-780C, then cooled in water to the ambient temperature. This operation of additional'annealing aims at providing coagulation of lamellar cementite present in pearlite as well as annealing the formed aluminium coating. The process course is shown in FIG. 3 illustrating the dependence of the rate of the heat treatment on the range of temperatures used.

Example 4. As a starting material for producing hotdip aluminized castings from a pearlitic malleable cast iron with spheroidal cementite, the non-aluminized castings of a black heart malleable cast iron may also be used. The said castings are surface dressed and aluminized with the same parameters as those the examples given above. Subsequently, they are annealed for a period of 0.25 10 hours at temperatures in the range of 650 800C. This process enables obtaining pearlite with spheroidal cementite and also formation of a homogeneous coating. The process is shown'in FIG. 4 illustrating the dependence of temperatures on the rate of the heat treatment.

Example 5. A process of the invention for producing hot-dip aluminized castings from a black heart malleable cast iron consists in a simultaneous heat treatment of castings from the starting white cast iron in order to obtain the ferriticor pearlite-ferritic structure with the isolated temper carbon and also to obtain an aluminium coating with the highest mechanical properties.

The aluminizing operation of the castings is carried out with the same parameters as the aluminizing shown in examples given above. Subsequently the aluminized castings are submitted to two-stage graphitizing annealing, where they are heated to the range of temperatures from 950 1 ,000C and maintained at this temperature for the period of 18 24 hours, resulting in decomposition of free cementite and bleaching of a temper carbon. Then the castings together with the furnace are cooled to a range of critical temperatures from 880 730C and at these temperatures they are maintained for the period of 30 60 hours, then cooled to the ambient temperature.

During the second stage of annealing, the eutectoid cementite contained in pearlite is decomposed what enables obtaining a pure ferritic or pearlite-ferritic structure of the matrix with the isolated temper carbon. Simultaneously with the graphitizing annealing, the previously formed aluminium coating is also annealed, which provides increase of its mechanical properties and particularly its plasticity. The process is shown in FIG. 5 which illustrates the dependence of the rate of the heat treatment on the temperature of this process.

What we claim is:

l. A method-of producing aluminized iron castings selected from the group consisting of aluminized pearlitic malleable iron castings with lamellar or coagulated cementite, and black heart malleable iron castings, said method comprising dipping into a hot bath containing aluminum or aluminum alloys, an iron casting which has not yet been subjected to graphitizing annealing, said iron casting being selected from the group consisting of white cast iron castings, black heart malleable cast iron castings, and pearlitic malleable cast iron castings, and heating the iron casting now provided with a protective aluminum coating from said bath to a temperature to effect graphitizing annealing of said iron casting and simultaneous annealing of the coating to provide homogeneity of the structure of the casting and the coating.

2. A method as claimed in claim 1 wherein the target cast iron is a malleable pearlitic cast iron with lamellar cementite obtained from white cast iron, the bath temperature being 680950C, the graphitizing annealing temperature being 900-1,000C and being applied for 8-16 hours, after which the casting and furnace in which it is inserted are cooled to 730-8 80C and then fast cooled to ambient temperature.

3. A method as claimed in claim 1 wherein the target cast iron is a malleable pearlitic cast iron with spheroidal cementite obtained from white cast iron, the bath temperature being 680950C, the graphitizing annealing temperature being 9001,000C and being applied for 8-16 hours, after which the casting and furnace in which it is inserted are cooled to 730-880C and then fast cooled to ambient temperature, and further comprising an additional annealing at a temperature of 650-780C for 0.25l0 hours.

4. A method as claimed in claim ll wherein the target cast iron is a malleable pearlitic cast iron with spheroidal cementite obtained from pearlitic malleable cast iron with a lamellar cementite, the bath temperature being 680950C, the graphitizing annealing temperature being 650750C and being applied for 0.25-10 hours, after which the casting is cooled in water to an ambient temperature.

5. A method as claimed in claim 1 wherein the target cast iron is a malleable pearlitic cast iron with spheroidal cementite obtained from black heart malleable cast iron, the bath temperature being 680-950C, the graphitizing annealing temperature being 650-800C and being applied for 0.25-10 hours, after which the casting is cooled in water to ambient temperature.

6. A method as claimed in claim 1 wherein the target cast iron is a ferritic or pearlite-ferritic cast iron obtained from black heart malleable cast iron, the bath temperature being 680950C, the graphitizing annealing temperature being 950l ,000C and being applied for l824 hours, after which the casting and furnace in which it is inserted are cooled to 730880C and maintained thereat for 30-60 hours and then cooled to ambient temperature.

7. In a process for manufacturing hot-dip aluminized pearlitic malleable cast iron products with lamellar or coagulated cementite and also black heart malleable cast iron products, which are protected against corrosion and high temperatures by a hot-dipaluminum coating, the improvement comprising:

a. hot-dipping castings of non-annealed white cast iron in a bath of aluminum or aluminum alloys at temperatures in the range of 650-950C, for a period of time which is necessary for the immersed castings to obtain the bath temperature, the rates of immersing and removing said castings being between 0.5 and l0m/min.,

b. annealing the thus aluminized castings under oxidizing conditions, at a temperature in the range of 9001,000C for a period of time of 40 hours to obtain graphitization of cementite and simulta neous homogenization of the aluminum coating black heart malleable castings. l= l= 

1. A METHOD OF PRODUCING ALUMINIZED IRON CASTINGS SELECTED FROM THE GROUP CONSISTING OF ALUMINIZED PEARLITIC MALLEABLE IRON CASTINGS WITH LAMELLAR OR COAGULATED CEMENTITE, AND BLACK HEART MALLEABLE IRON CASTINGS, SAID METHOD COMPRISING DIPPING INTO A HOT BATH CONTAINING ALUMINUM OR ALUMINUM ALOYS, AN IRON CASTING WHICH HAS YET BEEN SUBJECTED TO GRAPHITIZING ANNEALING, SAID IRON CASTING BEING SELECTED FROM THE GROUP CONSISTING OF WHITE CAST IRON CASTINGS, BLACK HEART MALLEABLE CAST IRON CASTINGS, AND PEARLITIC MALLEABLE CAST IRON CASTINGS, AND HEARTING THE IRON CASTING NOW PROVIDED WITH A PROTECTIVE ALUMINUM COATING FROM SAID BATH TO A TEMPERATURE TO EFFECT GRAPHITIZING NANEALING OF SAID IRON CASTING AND SIMULTANEOUS ANNEALING OF THE COATING TO PROVIDE HOMOGENITY OF THE STRUCTURE OF THE CASTING AND THE COATING.
 2. A method as claimed in claim 1 wherein the target cast iron is a malleable pearlitic cast iron with lamellar cementite obtained from white cast iron, the bath temperature being 680*-950*C, the graphitizing annealing temperature being 900*-1,000*C and being applied for 8-16 hours, after which the casting and furnace in which it is inserted are cooled to 730*-880*C and then fast cooled to ambient temperature.
 3. A method as claimed in claim 1 wherein the target cast iron is a malleable pearlitic cast iron with spheroidal cementite obtained from white cast iron, the bath temperature being 680*-950*C, the graphitizing annealing temperature being 900*-1,000*C and being applied for 8-16 hours, after which the casting and furnace in which it is inserted are cooled to 730*-880*C and then fast cooled to ambient temperature, and further comprising an additional annealing at a temperature of 650*-780*C for 0.25-10 hours.
 4. A method as claimed in claim 1 wherein the target cast iron is a malleable pearlitic cast iron with spheroidal cementite obtained from pearlitic malleable cast iron with a lamellar cementite, the bath temperature being 680*-950*C, the graphitizing annealing temperature being 650*-750*C and being applied for 0.25-10 hours, after which the casting is cooled in water to an ambient temperature.
 5. A method as claimed in claim 1 wherein the target cast iron is a malleable pearlitic cast iron with spheroidal cementite obtained from black heart malleable cast iron, the bath temperature being 680*-950*C, the graphitizing annealing temperature being 650*-800*C and being applied for 0.25-10 hours, after which the casting is cooled in water to ambient temperature.
 6. A method as claimed in claim 1 wherein the target cast iron is a ferritic or pearlite-ferritic cast iron obtained from black heart malleable cast iron, the bath temperature being 680*-950*C, the graphitizing annealing temperature being 950*-1,000*C and being applied for 18-24 hours, after which the casting and furnace in which it is inserted are cooled to 730*-880*C and maintained thereat for 30-60 hours and then cooled to ambient temperature.
 7. In a process for manufacturing hot-dip aluminized pearlitic malleable cast iron products with lamellar or coagulated cementite and also black heart malleable cast iron products, which are protected against corrosion and high temperatures by a hot-dip aluminum coating, the improvement comprising: a. hot-dipping castings of non-annealed white cast iron in a bath of aluminum or aluminum alloys at temperatures in the range of 650*-950*C, for a period of time which is necessary for the immersed castings to obtain the bath temperature, the rates of immersing and removing said castings being between 0.5 and 10m/min., b. annealing the thus aluminized castings under oxidizing conditions, at a temperature in the range of 900*-1,000*C for a period of time of 40 hours to obtain graphitization of cementite and simultaneous homogenization of the aluminum coating structure, and c. slowly cooling the castings in a furnace to a temperature in the range of 730*-880*C, then cooling the castings in water, oil or air to the ambient temperature in the case of pearlitic malleable castings, or soaking at temperatures of 730*-880*C for a period of 30-60 hours and subsequently air-cooling the castings to ambient temperature in the case of black heart malleable castings. 